They are paraffinoid in nature, the wax content being as high as 0%. The crude wax melts at about 40°C. but from it a series of waxes with melting poi its from to 73°C. has been obtained. The phenolic constituents resemble those of peat tar and are efficient bactericides. The basic components belong to the pyridine and quinoline series and are chiefly methylated derivatives. A distinctive characteristic of lignite tars is the presence of ketones in the neutral oils—a resemblance to the oils of wood tar. Lignite tars are of increasing importance as a source of fuels for internal combustion engines, burning and lubricating oils and pitch.
Low-temperature Coal Tars.—(a) Vacuum Tar has only been produced experimentally with the object of elucidating the composition of coal and the effect of heat upon this material. It is obtained when coal is carbonized at temperatures up to 45o° C. under a pressure of 5-4omm. and the average yield is 14 gallons (6.5% by weight) per ton of coal. It is usually lighter than water (sp.gr. 0.99) and is especially interesting in that some of its con stituents are identical with substances found in petroleum oils, and extractable from coal itself by solvents such as pyridine. It con sists of 40-45% paraffins and naphthenes, 40% "ethylenic" hy drocarbons richer in carbon than mono-olefines, 7% aromatic hydrocarbons, chiefly homologues of naphthalene, and io% phe nols, mostly cresols and xylenols, with smaller amounts of tertiary alcohols and basic compounds. Naphthalene, anthracene, benzene and its homologues are all absent.
Amongst the more interesting constituents of vacuum tar are the naphthene, called melene which is also found in Galician petroleum and in the distillate of bees' wax, and the hydrogenated phenols (alcohols) such as hexahydro-p-cresol and its homologues.
(b) Low-Temperature Tar is produced by the carbonization of bituminous coals, containing 28-35% volatile matter, at tempera tures not exceeding 650°C., which is an optimum temperature for the production of gas, tar and smokeless fuel (see CARBONIZA TION). It is essentially a mixture of those volatile products of coal which are liquid at ordinary temperatures and which have not been subjected to the secondary thermal decomposition incidental to high-temperature carbonization in horizontal retorts. It is therefore regarded as a "primary" tar. Many factors affecting the yield, composition and other characteristics of low-temperature tars have been varied in experimental plants with the result that there have been produced many types of tars to which it has been impossible to assign a common composition. Certain well defined characteristics may be shortly summarized of primary tars ob tained by low-temperature carbonization processes which have been commercially established.
Low-temperature tar is usually a brownish black oil, much less viscous than the tar made from the same coal at high tempera tures. It is obtained in an average yield of 16-18 gallons per ton of coal carbonized, or 8-9% by weight. This is approximately double the weight obtained by distilling the coal at Its sp. gr. is 1.02-1.07, and owing to the low temperature at which it is formed, it is mainly paraffinoid in character, whereas high temperature tar is almost wholly aromatic.
The most distinctive characteristics of low temperature tar are the almost entire absence of benzene and its homologues, naph thalene, anthracene and phenol; the unsaturated nature of the neutral oils, from which can be isolated solid paraffins; and the presence of solid, amorphous compounds, of either basic or phe nolic nature.
The crude tar is a most unstable product and begins to decom pose at about 200° C., finally yielding, when distilled to 360°C., an average of 65% oily distillate and 35% pitch. The amount of pitch formed on distilling low-temperature tar is therefore only half that obtained in a similar operation with high-temperature tar. The former, too, contains much less "free carbon" (i.e., mat ter insoluble in benzene) than the latter, the average amount being 2-4%. By modified extractions with caustic soda and sulphuric acid (see COAL TAR) the crude distillate can be divided into (i.) neutral oils, (ii.) phenols and (iii.) bases.
(i.) Neutral oils consist almost wholly of paraffins, naphthenes and olefines, with comparatively small amounts of methylated derivatives of naphthalene and anthracene. On treatment with concentrated sulphuric acid, the neutral oils are absorbed with the exception of some 12% consisting mainly of paraffins. Crude solid wax representing over 1% of the tar is obtained on cooling an acetone solution of the neutral oils. In the paraffin series, all the lower members have been identified, and in the higher ranges, the solid members from to The latter melt at 62°C. a- and 0-methylnaphthalenes have been isolated, but no naphthalene. Similarly (3-methylanthracene occurs in a complex mixture of hydrocarbons crystallizing on cooling the high fractions of the distillates, but anthracene is absent. No phenanthrene or carbazole has been found, and in place of the hydrocarbons fluor ene, acenaphthene and Biphenyl, there occur the fully hydroge nated products, perhydrofluorene, perhydroacenaphthene and dodecahydrodiphenyl.